Conventionally, as filter media for filters for the purpose of air purification, etc., an electrostatic filter medium including a nonwoven fabric made of polypropylene or like synthetic fibers, a mechanical filter medium made of glass fibers, and the like are known. However, an electrostatic filter medium has a problem in that the electrostatic performance is degraded due to oil mist or moisture in the gas, resulting in a decrease in collection efficiency. In addition, a mechanical filter medium made of glass fibers has a problem in that improvement in collection efficiency leads to an increase in pressure loss, and also has the problem of disposal.
In addition, a multilayer filter medium obtained by laminating nonwoven fabrics has been proposed (see, e.g., PTLs 1 to 5). Further, a filter medium for a filter using a nanofiber has been proposed (see, e.g., PTLs 6 and 7).
However, there has been a demand for a filter medium for a filter further improved in terms of low pressure loss and high collection efficiency. Further, in addition to low pressure loss and high collection efficiency, improvement in pleatability and wind-pressure deformation resistance has also been demanded.